Zero Oil Pressure, no other indications

I recently noticed that I have zero oil pressure. I have checked the oil many times and have good quantity. There are no other associated bells and whistles with the zero pressure indication. I plan on checking the sending unit but wondering why isnt there any other indication. Any suggestions?

It is an 08 silverado 4wd crew cab with 39K on it. No other lights or warnings. Oil quantity is good, no leaks. No heating problems, no knocking or other noises. There was absolutely no guage response to rpm changes for a while until yesterday when it just broke zero and fluctuated a small amount before falling back below zero.